What is a “High” Laser Beam Quality?

Posted on 2008-03-04 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2008_03_04.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on beam quality and broad-area laser diodes

The beam quality of a laser beam is commonly characterized with the beam parameter product or the M² factor. However, a beam which looks good in terms of these quantities may nevertheless create a real mess in an application.

As an example, it can happen that the emission pattern of a broad-area laser diode has a complicated shape, even though the M² factor is not that large. Even worse, the detailed shape can depend on the pumping conditions. If such a laser diode is used for pumping a solid-state laser (→ diode-pumped lasers), it induces a complicated pattern of thermal lensing in the laser crystal, where changes in the pump power lead to a change of the focal length and possibly also to a beam deflection, if the center of the pump beam also moves. All these effects may not matter that much if a 1-W laser is constructed with a crystal material having a high thermal conductivity, but for a glass laser it may be a real problem.

We learn that instead of simply looking at M² numbers, it is necessary to properly assess the requirements for some application, and then see whether M or something else is the right quantity for judging what is suitable.

Of course, a beam may also have a “nice” smooth intensity profile, while its focusability is really poor, as expressed by a large M² value. So one cannot judge beam quality (in the sense of focusability or smooth wavefronts) by simply looking at the intensity profile.